It is generally well known in the art to fabricate containers by the process of blow molding, wherein a plastic parison known as a “preform” is molded into a container. This preform is generally in the form of an elongated tube which defines a preform cavity and which is provided with a closed distal end and a mouth at an open proximal end in communication with said preform cavity. The preform may be fabricated from any polymer resin which has appropriate deformation characteristics; polyethylene terephthalate (PET) and polypropylene (PP) are particularly favored for their deformation properties and suitability for use with alimentary products.
The preform is first placed in a mold which defines a mold cavity in the form of a container. The mold is usually provided in at least two segments, ideally configured such that the open proximal end of the preform protrudes from the mold while a majority of the preform remains within the mold cavity. A pressurized fluid is then injected into the preform cavity, which induces it to expand and assume the contours of the mold, thereby forming a container.
When forming a container by the blow molding process, it is necessary to ensure that the segments of the mold are held together tightly at the end of the forming process where the preform is in contact with the interior of the mold and is exerting an outward force upon the mold surfaces. Otherwise, the force exerted by the preform on the surface of the mold cavity as it is formed will cause the mold segments to separate and result in the undesirable formation of prominent parting lines in the container.
In known embodiments of the blow molding process, the expansion of the preform is made by the injection of compressed air. To maintain tight closure of the mold segments, one or several compensation chambers, for instance pneumatic cylinders, are provided in one or more of the mold segments to press or clamp the mold segments together, the compensation chambers being fed from the same compressed air supply as the molding apparatus. Preferably, one mold segment is held stationary while the other is pressed against it by the pneumatic cylinder.
However, it has recently become known to make the expansion of the preform by the injection of a liquid, in particular the liquid which is ultimately to be packaged in the container thereby fabricated. In a blow molding apparatus so configured, there is thus no supply of compressed air, requiring an alternate means for ensuring the closure of the mold segments to be furnished so as to avoid the formation of parting lines on the containers being produced.
In particular, the international patent application publication WO 2012/0170517 describes a system wherein there is provided a means for driving a hydraulic cylinder placed in abutment to one of the mold segments, thereby ensuring the proper closure of the mold during the injection step. In one embodiment, the injection liquid and the hydraulic cylinder are bridged by an isolator device. The isolator device comprises a diaphragm or piston disposed in an injection head of the apparatus and acted upon directly by the injection liquid, thereby pressurizing the hydraulic fluid and driving the hydraulic cylinder. This embodiment is particularly appealing in that only one pressure source is employed. In an alternate embodiment, the hydraulic circuit is isolated from the injection liquid and driven by a separate pressure source.
However, the systems described in the prior art do not satisfactorily resolve the problem of ensuring the proper closure of the mold segments.
Specifically, the apparatus of the prior art reference is constrained in that the pressure for driving the compensation mechanism is limited by the pressure of injection of the liquid into the preform. Thus, in order to generate a sufficient force on the mold segments to hold the mold closed during injection, the principle of hydraulic force multiplication dictates that the area of the isolator upon which the pressure of the injection fluid acts must necessarily be greater than the area of the projection of the container onto the plane normal to the direction of motion of the mold segment. In practice, this means that the surface area of the plunger, piston, or diaphragm of the isolator must exceed the area of the longitudinal section of the container being formed for the compensation mechanism to provide enough force to keep the mold closed during the forming process. The isolator unit and injection head are very large and unwieldy as a result.
It is therefore an object of the invention to provide an apparatus which employs a single pressure source to ensure the closure of a mold during the blow molding of a container within, and which is compact and reliable.